1. Field of the Invention
The present invention relates to the purification of phenolic esters of aromatic carboxylic acids and more particularly to the purification of diphenyl esters of aromatic carboxylic acids, such as phthalic acid, isophthalic acid, etc., to provide ester monomers having good color properties.
2. Description of the Prior Art
The production of various phenolic esters of aromatic benzenedicarboxylic acids, such as diphenyl terephthalate, have become of significant commercial interest in recent years due to their use in a great many types of processes. As an example, diphenyl terephthalate and diphenyl isophthalate when dissolved in a solvent may be reacted with a primary diamine to produce polyamides. Likewise, 3,3'-diaminobenzidine may be condensed with various diphenyl esters to form polybenzimidazoles. In the synthesis of polyarylate resins, such as Durel.RTM., a mixture of iso- and terephthalates is reacted with bisphenol-A to provide resins which are of significant commercial interest. These esters may be produced by reaction of acid chlorides with a phenol to produce the phenyl ester and hydrogen chloride as a by-product, or by reacting a phenolic compound and aromatic dicarboxylic acid in the presence of a catalyst consisting of an alkali metal compound and a boron compound or organic titanates or zirconates. While such processes are effective for producing phenolic esters they suffer the disadvantage that during the process of esterification, the ester acquires a grey to brown color which methods of purification, such as vacuum distillation, recrystallization and/or carbon treatment, are insufficient to remove. Accordingly, there is a need for additional purification methods which are sufficient to achieve specification grade diphenyl phthalates.
U.S. Pat. No. 2,197,546 discloses color improvement of phthalic acid esters such as dibutyl, dihexyl, dioctyl, etc., phthalates, by heating the ester with salts of perboric acid at reflux under reduced pressure followed by separation of the ester by filtration. U.S. Pat. No. 2,780,643 discloses catalytic hydrogenation of synthetic alcohols for color improvement, followed by esterification of the alcohol with phthalic acid. U.S. Pat. No. 3,597,470 discloses purification of bis(2-hydroxyethyl) terephthalate for color improvement by contacting the terephthalate with a solution of sodium borohydride or diborane followed by crystallization and separation. U.S. Pat. No. 4,118,582 discloses purification of spent recycle ethylene glycol, recovered from polyethylene terephthalate manufacture, by adding an organic acid (e.g., acetic acid) and an alkali metal borohydride in the absence of oxygen to precipitate antimony oxide which is used as a catalyst in preparing the polyethylene terephthalate.
A specific purification method is disclosed in U.S. Pat. No. 3,705,186 for producing pure, colorless diphenyl terephthalate. It begins with transesterification of dialkyl terephthalates with at least equivalent amounts of phenyl acetate in the presence of butyltitanate as a catalyst by (a) heating the dialkyl terephthalate with phenyl acetate in an inert atmosphere in the presence of 1-5 wt. % of activated carbon to temperatures above 150.degree. C., (b) then adding the titanic acid ester, (c) immediately removing the alkyl acetate, (d) stirring the hot carbon-containing crude ester into a relatively high boiling aromatic hydrocarbon, such as xylene, under an inert gas, and (e) crystallizing the product after removing activated carbon by filtering.